The present invention relates to techniques permitting of modifying the electric charge of a space zone.
Processes are known for creating an electric charge of specific sign in a space zone, by favouring a concentration of ions of the same sign there. It is also known that in certain situations, in place of seeking to charge a space zone according to a specific polarity, it is useful to raise its degree of ionisation but not its overall charge, by simultaneously increasing its concentration with positive and negative charges.
The elimination of static electricity accumulated on an electrified body represents one case of application of this latter technique. In fact when such a body is plunged into a medium containing positive and negative charges, the electric field created by this body attracts the electric charges of contrary polarity which come to neutralize the charges accumulated on the body, and repels the charges of the same polarity as the electrified body.
The problem of elimination of electrostatic charges assumes great importance in various fields. Numerous bodies have in fact a tendency to accumulate positive or negative electric charges, either by influence, when they are disposed in an electric field, or under the effect of mechanical friction exerted between surfaces of different natures. When these bodies are insulators, or when they are conductive but not earthed, the charges tend to accumulate on these bodies in order to bring them to potentials which can sometimes reach extremely high values. These electrification phenomena are responsible for a certain number of damage effects which can be of mechanical order, for example sticking effects, or of electrical nature, such as the risks of electric shock for the personnel handling electrified bodies or the risks of sparking followed by explosion in inflammable media, the appearance of discharges which fog photographic films, etc.
Various types of electric charge eliminators based upon the principle of a combination of charges of a medium surrounding the body with charges of opposite sign accumulated thereon have already been proposed and utilised.
Among these devices mention may be made of radioactive eliminators which make use of the ionising properties of alpha and beta radiations to ionise slightly the air surrounding a body to be discharged. The efficacity of these devices is low by reason of the low degree of ionisation which one may hope to achieve without use of powerful radioactive sources (several tens of millicuries), the potential dangers of which, both as regards the risks of irradiation of the personnel and the risks of accidental dispersion of radioactive material, are not acceptable in numerous applications.
Corona effect eliminators also exist of the inductive type which are constituted by one or more conductive wires at earth potential fitted with points which are disposed in the proximity of the electrified bodies to be discharged. The high value of the electric field in the vicinity of the points favours the transference of charges between the electrified body and the eliminator.
Further corona effect eliminators make use of a high voltage electric source which creates an intense electric field in the vicinity of one or more points plunged into a gaseous medium in order to cause the formation of a corona discharge therein, generating ions. The produced high voltage is alternating so as alternately to produce positive and negative ions in the medium surrounding the electrified body to be neutralised.
It has however been observed that even these corona-effect eliminators suffered from inadequacies, and in certain cases could present dangers.
In particular it has been recognised that the devices utilised hitherto function effectively only when they are disposed in the immediate vicinity of the object to be discharged. Otherwise the formed ions tend to re-combine, by reason of their great mobility, before they have been able to come into contact with the body, this occurring the more rapidly as the level of ionisation which it is sought to create about the body is higher. It has further been observed that neutralisation was often imperfect or even in certain cases the body tended to acquire a charge of sign opposite to that which it had before the use of the eliminator.
Moreover the use of this type of eliminator must be banned in inflammable or explosive media since the corona discharges can give rise to sparks adapted to provoke ignition of the medium then in which they take place.
Finally it is known that corona discharges in air are accompanied by the formation of ozone, a highly oxidizing gas capable of deteriorating certain materials or presenting harmful effects for persons. This phenomenon is sometimes an obstacle to the use of corona-effect discharge eliminators.